Television transmitting system



De? 5, 1939. A. D. BLUMLr-:IN AL 2,182,578

TELEVIS-ION TRANSMITTING SYSTEM Filed Aug. 2, 1935 2 Sheets-Sheet l fi. Z). EL UML sm De@ 5, 1939. A. D. BLUMLEIN E'r AL 2,182,578

TELEVIS ION TRANSMITTING SYSTEM Filed Aug. 2, 1955 2 sheets-sheet 2 atented Dec. 5,V 1939 is STAT 'rnLEvrsroN 'ninemsn rannol SYSTEM Application August 2, 1935, Serial No. 34,304 f In Great Britain August 3, 1934 9 Claims. (Cl. 1781-12) The present invention relates to television ransmitting systems.

A television transmitting -system is known yin Y which an optical image of the object to be transaitted is projected upon a mosaic screen of moto-electrically active elements and the photolectric surface of the screen is scanned by a athode ray. The mosaic screen consists of a multiplicity of elements which are insulated from ach other and from a common signal plate, each lement forming, with the signal plate, a small ondenser. Between successive scans, each elenental condenser acquires a charge determined ny the number of photo-electrons emitted since he lastr scan, that is to say, a charge determined y the intensity of the light falling upon the elenent, and at each scan each elemental' condenser s discharged. Picture signals are derived, in an external circuit associated with the common sigial plate, from the electric impulses capacity- 'ed to the signal plate. if the following essential features:

(a) A mosaic screen having a large number of imall photo-electric elements, each element havng-capacity to a signal plate.A

`(b) Means for projecting an optical image of '.he object to be transmitted on to. these elenents.

(c) Means for collecting photo-electrons emitted by these elements, thus causing the :apacities to be charged (or discharged) in ac- :ordance with the brightness of the images.

(d) Switching means such as a cathode ray or )ther electrical switch" which periodically discharges (or charges) the condensers, bringing :he photo-electric elements to a definite potential, the discharging (or charging) current so produced developing the required picture signals.

Two arrangementspof the photo-electric elementsl are known. First the elements may be formed of small particles of metal insulated from a plate and lying on one side of the plate. :In this case, the light is thrown or to the same side of the plate-as that with which the cathode ray or electrical switch co-operates and such a` plate may be called Aa single sided mosaic screen. Secondly, thev elements may be formed on a grid or open workA plate from -which they arerinsulated, or may be formed of rivets projecting through a plate from which-they are insulated. The elements thus`app'ear at both sides of the plate and it may be arranged that the light image is thrown on one side and the cathode ray or other switch operates on the other side Used in this mannen-the' plate and elements may Such a system consists be spoken of as a double sided mosaic screen.

A television transmitting system employing a cathode ray tube having a single sided mosaic screen is described by Zworykin in the Journal of thelnstitution of Electrical Engineers of October 1933, page 437. In 4this system the optical image and the cathode ray operate on the same side of the mosaic screen. The cathode ray serves periodically to bring the potential of the mosaic elements to a few volts negative with respect to 10 the potential of an anode. This potential difference causes the photo-electrons emitted by the mosaic elements to iiow to the anode. With this device there are a number of difdculties which are caused by the fact that the collection lof the photo-.electrons depends on the. effect of the ray, by the iiuctuationsoiv the signals caused by sudden changesof illumination, by. the Iscattering of secondary electrons over the mosaic,

fully.v v Objects of the present invention are to effect improvements in the methods" of operating apparatus which includes a mosaic screen of the single-sided kind or ofthe double-sided kind and improvements in the Iarrangement and construc- Q tion of such apparatus.

Television transmitting apparatus has been proposed, as described in the specification of United States patent application Serial No. 19,752, iiled May 4, 1935, which comprises an optical system for projecting an image of an obect upon a photo-electrically active screen to cause 'emission of photo-electrons therefrom, a mosaicscreen comprising mutually insulated elements or being in the form 'of va lsheet having .Y a high resistance indirections parallel to its surface, the mosaic screen being spaced apart '40 from the photo-electrically active screen and being arranged with the mosaic screen within an evacuated envelope, focusing means for causing said photo-electrons to `form an electron may arise from the'iluctuations of the` signals caused by sudden changes of illumination, and it is a furtherfobject of the'present inventiom to overcome'thesrl diiculties.

According to the present invention, a method 9i television transmission. inwhichan optical w image upon the mosaic screen, means for developl; 5 s

' tential of the elements of the mosaic screen to approximately the potential of the cathode.

According to the present invention in a further aspect, television transmitting apparatus comprising a mosaic screen of photo-electrically active-elements insulated Vfrom one another and and it is preferably arranged that the total cafrom a common signal plate, means for projecting an optical image of an object to be transmitted on to said screen to cause emission of photoelectrons from said elements, an anode adapted to receive said photo-electrons, electrical switching means for scanning said screen by a beam of electrons, and an impedance electrically connected with said signal plate and across which the picture signal voltages to be transmitted are developed, is characterized by a circuit arrangement between said anode and said signal plate sucn that impulsive changes in average bright- Y ness of said image do not produce corresponding impulsive changes in the voltage across said impedance.

According to the invention in another aspect, television transmitting apparatus comprises a photo-electrically active screen, means for projecting an image of an object to be transmitted upon said screen'to liberate photo-electrons, an electrode for receiving said photo-electrons, either said screen or said electrode being of mosaic character and being arranged so that an electron image is formed thereon, means for scanning said mosaic to bring the potentials of Athe elements thereof to a datum value, a signal plate capacitively associated with said mosaic and, connected to said signal plate,` an impedance across which picture signal voltages are developed when the mosaic is scanned, characterised in that a circuit arrangement is provided for mixing with the signal voltage fromsaid impedance, before or after amplication thereof, a voltage which varies with the whole current due to said photo-electrons or with the alternating components of this current.

According to the invention in a further aspect,- television transmitting apparatus comprises a photo-electrically active screen, means for projecting an image of an object to be transmitted upon said screen to liberate photo-electrons, an electrode for receiving said photo-electrons, either said screen or said electrode being of mosaic character and being arranged so that an electron image is formed thereon, means Vfor scanning said mosaic to bring the potentials of the elements thereof to a' datum value, a signal plate capacitively associated with said mosaic and. connected to said signal plate, an impedance across which picture signal voltages are developed when the mosaic is scanned, characterised in that there is provided a circuit whereby the current due to said photo-electrons, or at least the alternating components of this current are fed to a point between said signal. plate and said impedance, g

Where the mosaic screen comprises mutually insulated metal elements, the direct capacity between adjacent groups of these elements, each i group constituting a picture dot, is less than the capacity of each group to the signal plate,

pacity of an the nements within the scannedi k area to the signal plate is such that the time constant, defined by this total capacity multiplied by the effective resistance of the cathode ray of the switching means, is not greater than the scanning is eiected with the aid of a cathode rayy the area of the beam is made of the same order as that of one picture dot.

The mosaic screen may be composed of small rivets' inserted in and insulatedffrom a common metallic supporting plate which serves as the signal plate, and the surface of the rivets on one side-may be made photo-electrically sensitive.

Apparatus according to the present invention preferably includes a multi-apertured plate disposed adjacent to the mosaic screen on the side thereof that is adapted tobe scanned bythe beam of electrons, and means for maintaining this plate at a potential positive with respect to the cathode which serves to'supply the beam of electrons.

Henceforth, the multi-apertured plate will be called a grid, The mosaic elements and the apertures inthe grid may have diierent pitches and/or congurations, as for example a triangular spaced set of mosaic elements co-operating with a rectangularly spaced set of apertures in the grid. Y

The potential of the grid relative to the mosaic surface is made so positive as to produce a strong electrostatic field normal to the mosaic surface, thus preventing any appreciable travel of electrons in a' direction aparallel to the surface of the mosaic screen. Furthermore, where the grid is associated with a single-sided mosaic screen, the grid may serverto collect the photo-electric emission from the elements.

The invention will'be further described with reference to the examples of parts of television transmission apparatus, shown in the accompanying diagrammatic drawings, in which similar parts in the diierent figures have the same reference numerals.

Fig. 1 shows a cathode ray tube having a mosaic screen of the double-sided type, with certain of the associated electrical circuits,

' Fig. 2 is a section, to a greatly enlarged scale, of a portion of the mosaic screen in Fig. 1 and a grid adjacent thereto,

Figs. 3 and 4 show respectively two modifications of a part of the arrangement shown in Fig. 1,

Fig. 5 shows acathode ray tube of the singlesided type, with certain of the associated electrical circuits, and

Fig. 6 is a section, to a greatly enlarged scale, of a portion ,of the mosaic screen shown in Fig. 5 and a grid adjacent thereto.

The apparatus shown in Figs. 1 and 2 comprises a cathode ray tube having an envelope in the form of a cylindrical portion I aring out into a bulbous portion 2. Within the envelope are arranged in the order mentioned, and starting from the closed end of the cylindrical portion l, a cathode 3 (the heating means for which are not shown), one or more electrodes, such as 4, which are adapted to take part in focusing the y trons. The cathode 3 and anode 5 may be of ray, an anode 5, a grid 6, a'mosaic dscreen 1 and an anode 8 which serves` to collect photo-elecany suitable kind and between the anode 5 and grid 6 there are disposed either electrostatic or electro-magnetic means for defiecting the ray so as to scan the mosaic screen 1 upon which the ray is focused. In the present example these means have the form of deecting coils 9 and I0.

The mosaic screen 1 is in the formof a metal signal plate II, provided with regularly arranged perforations and disposed normally to the mean direction of the cathode ray. The signal plate II carries elements in the form of rivets I2 passing right through the perforations in the plate. The rivets are insulated from the plate, for example by coating the plate, before the insertion of the rivets, with a layer I3 of insulating material, and the ends of the rivets on the face of the plate opposite the face scanned by the cathode ray are coated with photo-electric' material I4, such as caesium, the other ends ofthe rivets being uncoated.

The grid 6 is in the form of a plane, fine wire mesh disposed parallel to, and about l mm. from the mosaic screen, the cathode ray passing through this electrode on its way to the mosaic screen. The apertures in this grid should either register exactly with each mosaic element, so that each mosaic element has an aperture directly -opposite, as shown in Fig. 2, or else the'apertures of 1 the grid should have no correspondence at all with the mosaic elements. In order to achieve this latter arrangement, it is convenient to arrange that the pitch of the apertures in the grid is entirely different from the pitch of the mosaic elements, or that the arrangement of the apertures in the grid and the mosaic elements is on a definite basis. For example, the mosaic elements may be arranged in triangular spacing, whereas the grid apertures may be arranged ina rectangular system as would be possible by using wire gauze. Alternatively, the mosaic may be formed of a very large number of small particles in random arrangement co-operating with a comparatively widely spaced set of apertures in the grid. It is desirable, however, that the pitch of the apertures in the grid be ner thanthe distance between adjacent picture dots.

The photo-electron anode 0 may consist of a metal coating on the glass wall, or it may be in the form of a wire (r sheet metal electrode bent into rectangular or annular form. It is disposed at some distance from the mosaic screen in a plane parallel thereto.

A lens system I5 is provided, preferably outside the tube, as shown, for focusing an image of theV object to be transmitted upon' the photoeelectrically active surfaces I4 of the elements I2 of the mosaic screen, light from the object passing throughthe central hole in` the photo-electron anode 8.

The cathode of the tube is earthed by aconductor I6, 'the anode 5 is maintained at about 1000 volts positive with respect to the cathode by a source of potential difference I1 and the grid 6 is maintained at between volts and 500 volts positive with respect to the cathode by a source of potential difference I8.V

The signal plate II is connected by a conductor 26'to one terminal of a signal resistance I9 of 2000 ohms the other terminal of which is connected tothe negative terminal of a four-volt source .of current 20, the positive terminal of this source being earthed.

With the battery 2 I.

In order to ensure that the photo-electrons emitted by the mosaic elements` I 2 are all collected by the photo-electron anode 8, the negative end of a battery 2|, of 200 volts, is' connected to the terminal of the signal resistance I9 which is con-y nected `by the lead 26 to the signal plate II, and the positive end of this battery is connected to the photo-electron anode 8. A decoupling condenser 24, the capacity of which is large in relation to the total capacity of the mosaic elements I2 to the signal plate I I, is also connected in shunt The battery 2| must be well insulated from earth, and its capacity to earth must be kept'low.

In order to operate the device, the various members are switched on in the following order: cathode 3 and deflecting means 9 and I0, anode 5 and focusing electrode 4,' and iinally the photo-elem tron anode 8.

During operation an. image of the object to'be transmitted is projected by the'lens system I5 on to the face of the mosaic screen 1 and the back of the screen is scanned by the cathode ray.

Between the cathode 3 and anode 5, the cathode ray is accelerated to a velocity equivalent to 1000 volts, and, assuming for the moment that the parts of the mosaic screen 1 are at cathode p ctential, the ray is brought substantially to rest just as it reaches the mosaic screen.

Between successive scans, however, each ele'- ment I2, owing to the emission of photo-electrons, acquires a positive potential proportional to the intensity of the light falling upon the element. Therefore when the ray is deflected on to a positively charged element, the latter attracts the electrons of the ray. At each scan of an elementy therefore, the element is charged negatively by the scanning ray until it reaches approximately cathode potential. This potential may differ slightly from the potential of the cathode due to the temperature velocity of the emitted electrons and contact potentials etc. At this stage the'velocity of the ray is reduced to zero, so that an element cannot be charged. negatively.' The ray thus operates as a switch, reducing the potential of each element in turn almost instantaneously` to zero. Furthermore, provided the elements are not charged too highly, the velocity of the ray on i ments.l

The sudden changes of potential of the ele' ments I2 are capacity fed to the insulated signal plate Il and give rise to picture signals in the l signal resistance I9. These signals are tapped at 25 and amplified and transmitted in any known or suitable manner.

This device may be regarded as operating in the following manner. When an optical image is formed on the mosaic screen, a photo-electric current, which is representative of the instantaneous average intensity of illumination of the screen, flows from the mosaic elements I2 to the photo-electron anode 8 and through the circuit 2|, 26 to the sgnal`plate II. This may be regarded as the current charging -the condensers formed by the mosaic elements and the signalA plate, and it contains no picture signal component. VThus any impulsive componentsv of the photo-electric current are sent back from the photo-electron anode to. the signal plate, so that sudden changes of light,.which are much quicker than the picture period, produce no impulsive potential changes across the signal resistance I9,

Vthus capacity fed to the signal plate causes a ow of electrons `to earth through the signal resistance, the number of the electrons being proportional to the charge previously acquired by the y element. Thus the picturesignal currents, due to dot which are at a positive potential.

successive discharge of the mosaic elements, are unidirectional and are accordingly representative of the absolute intensity of illumination of the individual mosaic elements. In this Way, a1- though transient variations of intensity of illumination do not adversely influence the transmitted signals, low-frequency changes of intensity of illumination are adequately transmitted.

The purpose of the grid Gis two-fold. lIn the rst place it serves to screen the ray, over the greater part of its ilight, from the zero potential of the mosaic screen, and thus operates to preserve the focus of the ray. Secondly, the grid provides a strong potential vgradient normal to the signal plate. When the ray approaches an element at cathode potential, it is turned back Without striking the element. The strong potential gradient normal to the signal plate causes the electrons which have been stopped, to be accelerated back to the grid without any chance of their being pulled into elements of an adiacent This strong eld gradient thus prevents fogging" of the sigondary emissionA that may arise from the mosaic elements I2. 'I'his additional grid, whichisshown by dotted lines at 21 in Fig. 2, may be formed by a continuous mesh of electrically conducting` material deposited on the insulatinglayer I3 of the signal plate I I.

The mosaic screen may be constructed in the following manner. metal plate, about,0.l5mm. thick, is pierced'with-about 160,000 holes. 'Ihese holes may be made by etching the plate (preferably Irom both sides) so that the holes are countersunk. Alternatively, the holes may be made mechanically. The holes may conveniently be 0.2 mm. in diameter and `for a picture'of 200 lines, Vfour of such holes co-operate in forming what is referred to above as a picture dot. 'I'he plate is insulatedallbver by a thin layer of a suitable insulating material, such as glass, having a thickness of at least 0.2 mm. and the holes in the plate are then. lled with metal rivets. These can be formed in position by plating through from one side of -the plate to the other. The surfaces of these rivet elements are coated with silver on one side (for example by plating). 'I'he silver surfaces are then oxidised and activated with caesium' to form photo-electric elements.

With such a construction, the direct capacity between adjacent elements is much smaller than the capacity of an element to the signal plate.

4 Taking these dimensions and assuming a dielectric constant of 3 for the insulating material, the capacity per picture'dot (four elements) is approximately 0.5 micro-microfarad. For a picture consisting of 200 lines, each of 200 dots, making 40,000 dots to the complete picture, the total capacity of all elements to the signal plate is 20,000 micro-microfarads.

In order that moving pictures may be satisfactorily represented, it is necessary that the cathode ray shall be capable of discharging this total capacity substantially during `a complete scanning cycle. By a complete cycle is meant a cycle during which the ray occupies all possible positions and such a cycle would include more than one frame period of a scanning cycle employing interlaced scanning. If for example the complete scanning cycle occupies a 25th of a second, it is necessary that the time constant of discharge of this capacity shall be less than a 25th of a second, i. e. the ray resistance should be such that the 'product of the ray resistance and 20,000 micro-microfarads will be less than tr, second, that is to say, the ray resistance should be less than two megohms and preferably should not exceed one megohm.

This requirement of the product of the ray resistance and mosaic capacity can be looked at from another point of view. In the example given above, 40,000 picture dots were scanned in a 25th of a second, so that the ray rested on each picture dot for one millionth of a second. During.

this period it is necessary that the capacity of one picture dot viz. 0.5 micro-microfarads, shall be substantially discharged. Hence the product of the ray resistance'and the dot'capacity must be less than one millionth of a second, that is, the dot time. For the example taken, this again leads .to a requirement of a ray resistance less than two megohms and preferably one megohm.

By ray resistance is meant the etective resistance of the cathode ray or other electrical switch looked upon as a switching resistance.' If the elements of the mosaic depart from their datum voltage by a voltage V. and such voltage departure produces a re-stabilizing current equal to I, then the ray resistance is given by the ratio The moment the ray'hits the mosaic element,

the voltage V is rapidly reduced, butfor any 'value V of the mosaic element, va certain *resultant number of electrons corresponding to the current AI will be attracted by the elements.- Suppose in the above example the mosaic elements were al1 short circuited to the signal plate, and the signal plate were held volts more positive than the normal datum voltage of the mosaic elements (approximately cathode voltage), then a certain charging current would be obtained from the cathode ray. If this charging current were 10 microamps, then the. average ray resistance over the range of 0 to l0 volts would be 10 volts l0 mlcroamps which is one megohm.

The light intensity falling on the mosaic elements must never be so great that a very large positive potential is developed on the elements during the period between successive scannings of the beam. If such a large potential is developed, the electrons hitting the mosaic elements will liberate secondary electrons, which may be more numerous thanthe primary electrons, and thus prevent the-ray` from restoring the mosaic elements toapproximately cathode potential.

rIn the example shown in Fig. 1, it is desirable ;o limit the rise in voltage of the mosaic elenents to between and 20 volts positive with 'espect to the cathode ray cathode. Similarly, it s important that, during the process of switchng on the tube, the elements should not attain a. high positive potential. Suppose for example :hat before the cathode ray is switched on the chato-electron anode is energised and light falls Jn the mosaic, the elements will become positively charged to an abnormal extent, and not only will the ray when switched on be unable to restore them to cathode potential, but there is also a risk of breaking down the insulation of the elements to the signal plate. To prevent this, a suitable order of switching on the various potentials should be observed. For example, the cathode 3 and the current'to the defiecting coils 9 and I0 may be switched on iirst. After that, the potentials are applied to the anode 5, then the grid 6 and iinally the photo-electronanode 8.

The provision of the battery 20, which holds the signal plate slightly negative relative to the cathode, ensures that any leakage in the insulation of the mosaic elements shall tend to hold these elements negative relative to the cathode, rather than allow them to drift to a dangerously high positive Voltage. Similarly, this arrangement has the advantage that, if the insulation of any element tothe signal plate breaks down, this element is held negative and becomes inoperative. spot on the received picture, whereas had the signal plate been positive, a very bright spot might have been produced.

The presence of the grid materially increases the capacity of the mosaic screen to earth. Ii the mosaic and grid are each 10 cm. square and are separated by 1 mm. as described above, the eective capacity of the signal plate to earth is of the order of 100 micro-microfarads. In order to reduce the abnormally high noise-tosignal ratio which would be produced by working this device into a single ordinary valve, the device may be worked into several valves in parallel, it being arranged that the input capacity of the valves equals the eiective capacity of the device.

As an alternative, the grid may be more widely spaced from the mosaic screen, thus reducing the increase in capacity. A

The grid must not be placed near enough, and

`its potential must not be high enough, to extract electrons from the mosaic screen. Also the tube must be highly evacuated in order to prevent.

trouble arising from the presence of ions.

In the modiiication shown in Fig. 3,the negative terminal of the battery 2 I, which maintains the photo-electron anode 8 at a suitable positive potential with respect to the signal plate and the cathode, is connected to earth instead of to the signal plate. The positive terminal of this battery is connected to the'anode 8 through al resistance 23, while the condenser 24, as in Fig. l, is connected between the anode 8 and the signal plate. With this modied arrangement, the alternating component of the current owing to the photo-electron anode 8 is fed through the condenser 24 to a point in the lead 2B which .connects the signal plate to the signal resistance. Howeven the direct component of this current in this case passes to earth through the re sistance 23 and the battery 2l.

In the modication shown in Fig. 4, the conpotential.

This will causera slightly darker i denser 24 of Fig. 3 is omitted, with the result that the alternating component of the current due to the photo-electrons` passes through the signal resistance I9. A voltage proportionalto the photo-electroncurrent is developed across the resistance 23. The changes in potential difference across this resistance are fed to an ampliner 3|, While the picture signals are fed to an amplier 28. The outputs from the ampliers 3| and'28 are fed to a mixing device 29 so arranged that the component of the picture signal, due to the alternating component of thephoto-electric current passing through the signal resistance I9, is neutralized by the output from the amplifier 3|. Thus the signals developed in the mixer output leads il) are free from effects due to transient changes inintensity of illumination ofthe object being transmitted.

The devices shown in Figs. 1 to 4 employ a double sided mosaic screen, the mosaic elements of which are periodically restored to cathode The advantage of neutralising the effects of transient changes of illumination, may be obtained with such a mosaic, even ifv the mosaic elements are periodically restored to some potential approximating to the potentialof the anode of the device emitting the beamof electro'ns, in the manner described by Zworykin in the article referred to above.

Similarly the improved arrangement whereby the mosaic elements are periodically restored to cathode potential may be employed with a singlesided mosaic screen, as shown in Figs. 6 and 7. In this case the tube is similar to that shown in Fig. 1, except that themosaic screen 1 is inclined tothe electron gun 3, 4, 5 to allow .the optical image to be thrown on the same side as that on Which the cathode ray falls, and the lens system |15 is placed on the opposite side of the screen. No separate photo-electron anode is provided; the grid 6 acts as the photo-electron anode, since its potential is positive with respect to the maximum potential acquired by the mosaic elements.

The wires of the grid 6 must be of extremely small diameter, since the image is projected through the grid. The mosaic screen 1 includes a mica sheet |3a (Fig.' 6). Onthe side of this atv sheet facing the electron gun is formed a mosaic of separate silver elements lila which are oxidised and photo-sensitized with caesium. On the opposite side of the mica sheet is a continuous silver signal plate Ha. In this arrangement it is d'ei sirable to limit to a low value the maximum potential to which the mosaicl elements may rise,

vsince the photo-active surface mayemit secondary electrons at quite low voltage bombardment, Also it is desirable to make the mica sheetA |3a comparatively thick in order `to keep the capacity of the mosaic' elements to the' signal plate sumciently low.

We claim:

1. Television transmitting apparatus comprising a mosaicscreen which includes a signal plate and a multiplicity of photo-electrically active mosaic elements insulated from one another 'and from said signal plate, means for projecting an optical image of an object to be transmitted on to said screen to cause emission of photoelectronsl from said elements, an anode for 'receiving said photo-electrons, electrical switching means for scanning said screen by a beam of electrons, an impedance electrically connected with said signal plate and across which thepicture signal voltages to be transmitted are developed, and a circuit which connects said an- Y ing a photo-electrically-active screen electrode,

vas

Ameans for projecting an image of an object to be transmitted upon said screen electrode to liberate photo-electrons, an electrode for'receiving said photo-electrons, one of said electrodes being of` mosaic character and being arranged so that an electron image is formed thereon, means for scanning said mosaic electrode to bring the potentials of diierent parts thereof successively to a datum value, a signal plate capacitively associ-ated with said mosaic electrode, an impedance connected to said signal plate across which picture signal voltages are developed when said mosaic electrode is scanned, and a circuit forv mixing with the signal voltage from said impedance a voltage which'varies with at least the alternating components of the whole current due to said photo-electrons.

3. Television transmitting apparatus comprising a photo-electrically-active screen electrode. means for projecting an image of an object to be transmitted upon said screen electrode to liberate photo-electrons, an electrode for receiving said photo-electrons, one of said electrodes being of mosaic character and being arranged so that an electron image is formed thereon, means for scanning said mosaic electrode to bring the potentials of diierent parts thereof successively to a datum value, a signal plate capacitively associated withksad mosaic electrode, an impedance connected to said signal plate across which picture signal voltages are developed when said mosaic electrode is scanned, and a circuit for feeding at least the alternating components of the current due to said photoelectrons to a point between said signal plate and said impedance.

4. Television transmitting apparatus comprising a mosaic screen having groups of mutually insulated photo-electrically-active mosaic elements, each group constituting a picture dot, a signal plate adjacent to and insulated from said elements, means for projecting on to said screen an optical image of an object to be transmitted, and electrical switching means which include a cathode for supplying a beam of electrous. means for periodically deflecting said beam over and thereby scanning said screen, the total capacity to said signal plate of all of the said mosaic elements within the area of said screen `scanned by said beam is such that the time constant, defined by this total capacity multiplied by the effective resistance of said cathode beam, is not greater than the period to be occu- -pied by one complete scan of said area, an elec- Y trode for receiving photo electrons from the photo-electrically active mosaic elements, an impedance electrically connected with said signal plate across which the picture signal voltages to be transmitted are developed, and means for connecting said electrode to said signal plate so that at least the alternating components of the whole current due to said photo electrons may be applied across said impedance. A

5. Television transmitting apparatus comprising a mosaic-screen having a signal plate and photo-electrically-active mosaic elements insulated from one another and from said signal plate, means for projecting on to said screen an optical image of an object to be transmitted. electrical switching means which include an electron gun having a cathode for producing a beam of electrons and means for periodicallyA dellecting said beam. over and thereby scanning said screen, a grid disposed adjacent to said mosaic screen and on the side thereof that is adapted to be scanned by said switching means, and means for maintaining said grid at a potential positive with respect to the potential of said cathode, the apertures of the grid being arranged to register exactly in number and size with said mosaic elements.

6. Television transmitting apparatus comprising a mosaic screen having a signal plate and photo-electrically-active mosaic elements insulated from one another and from said signal plate, means for projecting on to said screen an optical image of an object to be transmitted, electrical switching means which include an electrongun having a cathode for producing a beam of electrons and means for periodically deilecting said beam over and thereby scanning said screen, and means for maintaining said signal plate at a potential which is slightly negative with respect to the potential of said cathode.

7. A television transmitting apparatus comprising a metallic signal plate provided with apertures, metal mosaic elements located in said apertures and insulated from said signal plate,.

the ends of said elements on one side of said plate being coated with photoelectrically active material and the otherA ends of said elements being relatively poor emitters of secondary electrons, means for projecting an optical image on l to the side of said plate on which said photoelectrically active material is exposed, electrical switching means including a cathode for scanning the other ends of said elements with a beam of electrons, an anode for collecting photoelectrons emitted by said elements, a signal impedance, an-electrical circuit including said impedance and said signal plate, and means to apply at least the alternating components of lthe current due to said photo-electrons to a point between said signal plate and said impedance.

8. A television transmitting apparatus comprising a mosaic screen having a signal plate and photo-electrically active mosaic elements insulated from one another and from the signal plate and cooperatively associated therewith, means for projecting onto said screen an optical image of an object to be transmitted, an electron gun structure including a cathode and an accelerating anode, means for maintaining said anode at a positive potential with respect to said cathode for producing a beam of electrons, means for periodically dellecting the beam of electrons vover said screen to scan the elements thereof, an

electrode positioned adjacent said mosaic screen and on the side thereof adjacent the gun structure, and means for maintaining said electrode at a positive potential with respect to the signal plate and the electron gun cathode and materially negative with respect to said accelerating anode, whereby the beam of electrons will be reduced to substantially zero velocity at the Scanned surlace of the mosaic screen;

9. A television transmitting apparatuscomprising a cathode ray tube including a gun structure having a cathode and an accelerating anode, means for maintaining said anode at a positive potential with respect to said cathode to produce a beam of electrons, a mosaic screen in said tube, said screen including a signal plate and a plurality of discrete light responsive mosaic elements positioned in cooperative relaonship with the signal plate, beam deection means for causing the beam of electrons' tol scan the light resplonsive elements `of the mosaic screen, means to`project an optical image of an 'object to be'transmitted upon said screen to l0 substantially the same potential as the cathode of ama,

the electron gun structure, and means for maintaining said grid electrode at a positive potential with respect to the signal plate and materially negative with respect to the potential of` the accelerating. anode, whereby the velocity of the beam of electrons will be reduced to substantially zero at the scanned surface of the mosaic screen.

JAMES DWYER MOGEE. 

